Thin film transistor, method of fabricating the same, and method of fabricating liquid crystal display device having the same

ABSTRACT

A thin film transistor includes a gate electrode, a gate insulation layer on the gate electrode, source and drain electrodes formed on the gate insulation layer, a polysilicon channel layer overlapping the ohmic contact layers and on the gate insulation layer between the source and drain electrodes, ohmic contact regions over the source and drain electrodes for contacting the polysilicon channel to the source and drain electrodes, and doping layers over the source and drain electrodes.

This is a divisional application of application Ser. No. 11/802,541,filed on May 23, 2007 now U.S. Pat. No. 7,754,548, which is herebyincorporated by reference. This invention claims the benefit of KoreanPatent Application No. 10-2006-057742 filed in Korea on Jun. 27, 2006,and Korean Patent Application No. 10-2006-058356 filed in Korea on Jun.28, 2006, which are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to a transistor, and moreparticularly, to a thin film transistor (TFT), a method for fabricatingthe same, and a method of fabricating a liquid crystal display devicehaving the same. Although embodiments of the invention are suitable fora wide scope of applications, it is particularly suitable for forming achannel layer of the TFT through an inkjet process.

2. Description of the Related Art

Generally, an active matrix liquid crystal display (active matrix LCD)that is widely used in a variety of image displays uses a thin filmtransistor (TFT) as a switching element. A semiconductor layer of thethin film transistor is formed of amorphous silicon. The use of theamorphous silicon is advantageous in fabricating a small-sized TFT LCD.However, since the amorphous silicon has low mobility, it is difficultto use amorphous silicon in fabricating TFTs of a large-sized TFT LCD.

Research for using a polysilicon layer having superior mobility as thesemiconductor layer in a TFT has been active. Since polysilicon can beeasily used in fabricating TFTs of the large-sized TFT LCD and a driveintegrated circuit (IC) can be formed on the substrate on which the TFTsare arrayed, integration can be improved and the fabrication costs canbe reduced. Methods of forming the polysilicon layer include directlydepositing the polysilicon and crystallizing the polysilicon afteramorphous silicon is deposited. Typically, the latter is widely used.That is, after the amorphous silicon layer is formed on a substrate anda crystallizing process is performed to change the amorphous siliconlayer into a polysilicon layer.

A polysilicon TFT includes a gate electrode and a source/drainelectrode, which are insulated from each other by insulation layers toindependently operate. The insulation layer is usually formed of aninorganic insulation material, such as silicon nitride (SiNx) or siliconoxide (SiOx) that has excellent manipulation property and excellentadhesion to metal.

FIG. 1 is a cross-sectional view of a TFT fabricated in accordance withthe related art fabrication method. As shown in FIG. 1, a TFT is formedby forming a buffer layer 2 functioning as an insulation layer on asubstrate 10 and subsequently forming an amorphous silicon (a-Si) layeron the buffer layer 2. A plasma-enhanced CVD process using SiH₄ gas, alow-pressure CVD process, or a sputtering process at a temperature ofabout 300-400° C. is used to deposit an organic insulation material,such as SiNx or SiOx. After the amorphous silicon layer is formed on thebuffer layer 2, an annealing process using an excimer laser is performedto crystallize the amorphous silicon layer into a polysilicon layer,after which the polysilicon layer is patterned into a channel layer 4.Subsequently, the organic insulation material, such as SiNx or SiOx, isdeposited over the substrate 10 to cover the channel layer 3, therebyforming a gate insulation layer 5.

Next, a conductive material, such as aluminum (Al) or an Al alloy, isdeposited over the gate insulation layer 5 and is patterned through aphotolithography process, thereby forming a gate electrode 1 on thechannel layer 4. Subsequently, N-type impurities are doped using thegate electrode 1 as a mask, thereby forming an ohmic contact layer 6 onthe channel layer 4. At this point, an ion doped region is a regionwhere source/drain electrodes 9 a and 9 b are formed and a portion ofthe channel layer 4 under the gate electrode 1 where the impurities arenot doped by becomes a channel region.

Next, an organic insulation material, such as SiNx or SiOx is depositedover the substrate 10 on which the gate electrode 1 is formed, therebyforming an inter-insulation layer 7. The deposition of the organicmaterial for the inter-insulation layer 7 is performed by a methodidentical to that for forming the gate insulation layer 5. Subsequently,contact holes are formed by etching portions of the inter-insulationlayer 7 and the gate insulation layer 5.

A metal layer is formed on the substrate 10 on which the contact holesare formed and is etched to form the source/drain electrodes 9 a and 9b, thereby completing the polysilicon TFT. However, according to theabove-described related art TFT fabrication method, since the maskingprocess is necessary, the fabrication process is complicated.Especially, since the channel layer and the ohmic contact layer formedthrough the ion doping process are formed through independent processes,the fabrication process is further complicated and the fabrication costsare increase.

SUMMARY OF THE INVENTION

Accordingly, embodiments of the present invention are directed to a TFT,a method of fabricating the TFT, and a method of fabricating an LCDdevice having the TFT that substantially obviate one or more problemsdue to limitations and disadvantages of the related art.

An object of embodiments of the invention is to provide a TFT having apolysilicon channel with a reduced number of processing steps.

Another object of embodiments of the invention is to provide a TFThaving a polysilicon channel at a low processing temperature.

Additional features and advantages of embodiments of the invention willbe set forth in the description which follows, and in part will beapparent from the description, or may be learned by practice ofembodiments of the invention. The objectives and other advantages of theembodiments of the invention will be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof embodiments of the invention, as embodied and broadly described, athin film transistor includes a gate electrode, a gate insulation layeron the gate electrode, source and drain electrodes formed on the gateinsulation layer, a polysilicon channel layer overlapping the ohmiccontact layers and on the gate insulation layer between the source anddrain electrodes, ohmic contact regions over the source and drainelectrodes for contacting the polysilicon channel to the source anddrain electrodes, and doping layers over the source and drainelectrodes.

In another aspect, a method of forming a thin film transistor includesforming a gate electrode on a substrate, forming a gate insulation layeron the gate electrode, forming source and drain electrodes on the gateinsulation layer, forming doping layers over the source and drainelectrodes, and forming a polysilicon channel layer on the gateinsulation layer between the source and drain electrodes and ohmiccontact regions contacting the polysilicon channel to the source anddrain electrodes.

In yet another aspect, a method of forming a thin film transistorincludes forming a gate electrode on a substrate, forming a gateinsulation layer on the gate electrode, forming source and drainelectrodes on the gate insulation layer, forming doping layers over thesource and drain electrodes, providing a liquid-phase silicon layer inbetween the source and drain electrodes and on the doping layers througha coating process, and annealing the liquid-phase silicon layer to forma polysilicon channel layer and ohmic contact regions for contacting thepolysilicon channel to the source and drain electrodes.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a cross-sectional view of a TFT fabricated in accordance withthe related art fabrication method;

FIGS. 2A through 2E are cross-sectional views illustrating a method offabricating a TFT according to a first embodiment of the presentinvention;

FIGS. 3A through 3E are cross-sectional views illustrating a method offabricating a TFT according to a second embodiment of the presentinvention;

FIGS. 4A through 4C are cross-sectional views illustrating a method offabricating a TFT according to a third embodiment of the presentinvention;

FIGS. 5A through 5C are cross-sectional views illustrating a method offabricating a TFT according to a fourth embodiment of the presentinvention;

FIGS. 6A through 6C are cross-sectional views illustrating a method offabricating a TFT according to a fifth embodiment of the presentinvention;

FIG. 7 is a top plan view of a pixel structure of a liquid crystaldisplay device according to the first embodiment of the presentinvention;

FIGS. 8A through 8F are cross-sectional views taken along line I-I′ ofFIG. 7, illustrating a method of fabricating the liquid crystal displaydevice of FIG. 7;

FIG. 9 is a top plan view of a pixel structure of a liquid crystaldisplay device according to the second embodiment of the presentinvention; and

FIGS. 10A through 10G are cross-sectional views taken along line II-IFof FIG. 9, illustrating a method of fabricating the liquid crystaldisplay device of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIGS. 2A through 2E are cross-sectional views illustrating a method offabricating a TFT according to a first embodiment of the presentinvention. As shown in FIGS. 2A and 2B, a metal layer is formed on atransparent insulation substrate 100 and etched to form a gate electrode101, after which a gate insulation layer 102 is formed on the insulationsubstrate 100 covering the gate electrode 101. The gate insulation layer102 is an inorganic insulation layer, such as SiNx or SiOx.

Next, a metal layer is formed on the gate insulation layer 102 formed onthe insulation layer and a doping layer is formed on the metal layer.The doping layer may be a phosphor-silicate-glass (PSG) layer,boro-silicate-glass (BSG) layer or an amorphous silicon layer doped withN+ ions or P+ ions. After the metal layer and the doping layer areformed on the gate insulation layer 102, as shown in FIG. 2B,photoresist is deposited on the doping layer. Then, source electrode 103a, drain electrode 103 b and doping layers 104 are simultaneously formedby etching the metal and doping layers in accordance with a maskprocess. Accordingly, the doping layers 104 are each formed entirelyover the source and drain electrodes 103 a and 103 b.

As shown in FIG. 2C, after the source and drain electrodes 103 a and 103b are formed on the gate insulation layer 102, a liquid-phase siliconlayer 105 is formed over and in between the source and drain electrodes103 a and 103 b through a coating process, such as an inkjet method. Theliquid-phase silicon layer 105 is formed from a silicon containingliquid-phase material, such as SixH₂x (CyclopentaSilane).

After the liquid-phase silicon layer 105 is formed in the channel regiondefined between the source and drain electrodes 103 a and 103 b, anannealing process is performed to form a polysilicon channel layer 106on the gate insulation layer 102 between the source and drain electrodes103 a and 103 b and overlapping the doping layers 104, as shown in FIG.2D. As the annealing process is performed, a thickness of theliquid-phase silicon layer 105 is reduced such that a height of thechannel layer 106 above the gate insulation layer 102 becomes a littlemore than that defined by the source and drain electrodes 103 a and 103b. The annealing process is performed by heating the substrate up to atemperature of 200-800° C. (540° C.) and irradiating a laser having awavelength of 308 nm and an energy of 345 mJ/cm2. In more detail,solvent contained in the channel pattern is removed through the heatingprocess and thus a thickness of the channel pattern is reduced. Further,the silicon is changed into polysilicon by irradiating the laser whiledopants from the doping layers 104 diffuse into the source and drainelectrodes 103 a and 103 b. However, embodiments of the presentinvention are not limited to this configuration. The heating temperatureand the energy of the laser may vary in accordance with a degree ofcrystallization, or a size of the LCD device and a material property ofthe liquid-phase silicon.

After the channel layer 106 is formed, a passivation layer 109 is formedon the insulation substrate 100 and etched to expose the source anddrain electrodes 103 a and 103 b, as shown in FIG. 2E. The passivationlayer 109 may be a SiNx-base inorganic layer or an acryl-based organiclayer. Subsequently, a metal layer is formed on the passivation layer109 and patterned to form terminals 107 a and 107 b that electricallycontact the source and drain electrodes 103 a and 103 b, respectively.

The method of fabricating the TFT according to the first embodiment hasan advantage of forming the polysilicon channel layer without performingdeposition and mask processes. Further, since the channel layer isformed without performing a PECVD process, the process load can bereduced.

FIGS. 3A through 3E are cross-sectional views illustrating a method offabricating a TFT according to a second embodiment of the presentinvention. As shown in FIGS. 3A and 3B, a metal layer is formed on atransparent insulation substrate 200 and etched to form a gate electrode201, after which a gate insulation layer 202 is formed on the insulationsubstrate 200 covering the gate electrode 201. The gate insulation layer202 is an inorganic insulation layer, such as SiNx or SiOx.

Next, a metal layer is formed over the gate insulation layer 202. Then,source and drain electrodes 203 a and 203 b are formed through aphotolithography process. After the source and drain electrodes 203 aand 203 b are formed on the gate insulation layer 202, a liquid-phasesilicon layer 204 is formed over and in between the source and drainelectrodes 203 and 203 b. Then, a doping layer 205 is formed on theliquid-phase silicon layer 204, as shown in FIG. 3C. The liquid-phasesilicon layer 204 is formed of a silicon containing liquid phasematerial, such as SixH₂x (CyclopentaSilane) and the doping layer 205 maybe a PSG (Phosphor-Silicate-Glass) layer, a boro-silicate-glass layer,or an amorphous silicon layer doped with N⁺ or P⁺ ions. After theliquid-phase silicon layer 204 and the doping layer 205 are formed,photoresist is deposited on the doping layer 205, after which a halftonepattern 280 is formed at a channel region, defined between the sourceand drain electrodes 203 a and 203 b over the gate electrode 202,through a diffraction mask or halftone mask process.

After the halftone pattern 280 is formed on the doping layer 205, anetching process is performed to form a channel pattern 204 a and dopinglayers 205 a and 205 b that are respectively overlapping the source anddrain electrodes 203 a and 203 b on the channel pattern 204 a, as shownin FIG. 3D.

Subsequently, as shown in FIG. 3E, an annealing process and a contactlayer forming process are performed using a laser to form a polysiliconchannel layer 206 a on the gate insulation layer 102 between the sourceand drain electrodes 203 a and 203 b and to form ohmic contact layers206 b at the edges of the polysilicon channel layer 206 a contacting thesource and drain electrodes 203 a and 203 b. That is, according to theTFT fabrication method of the second embodiment, the polysilicon channellayer 206 a and the ohmic contact layer 206 b are simultaneously formedthrough a single process. The annealing process and the contact layerforming process are performed by heating the substrate up to atemperature of 200-800° C. (about 540° C.) and irradiating a laserhaving a wavelength of 308 nm and an energy of 345 mJ/cm2. In moredetail, solvent contained in the channel pattern is removed through theheating process and thus a thickness of the channel pattern is reduced.Further, the silicon is changed into polysilicon by irradiating thelaser. In addition, during the above process, the doping layer isvertically diffused into the channel pattern to form the ohmic contactlayers 206 b.

After the channel layer 206 a and the ohmic contact layers 206 b areformed on the insulation substrate 200, although not shown in thedrawings, a passivation layer (insulation layer) is additionally formedon the insulation substrate 200, after which a contact hole formingprocess for exposing the source and drain electrodes 203 a and 203 b isperformed. Subsequently, a metal layer is formed on the insulationsubstrate 100 and patterned to form power terminals that electricallycontact the source and drain electrodes 203 a and 203 b, respectively.

The method of fabricating the TFT according to the second embodiment hasan advantage of simultaneously forming the channel layer and the ohmiccontact layer after the channel layer region and the ohmic contact layerare patterned using the halftone pattern (see FIG. 2E). Further, sincethe channel layer is formed without performing a PECVD process, theprocess load can be reduced.

FIGS. 4A through 4C are cross-sectional views illustrating a method offabricating a TFT according to a third embodiment of the presentinvention. As shown in FIG. 4A, a metal layer is formed on a transparentinsulation substrate 300 and etched to form a gate electrode 301, afterwhich a gate insulation layer 302 is formed on the insulation substrate300 covering the gate electrode 301. The gate insulation layer 302 is aninorganic insulation layer, such as SiNx or SiOx. Next, a metal layer isformed over the gate insulation layer 302 and a doping layer is formedon the metal layer. The doping layer may be a PSG layer, a BSG, or anamorphous silicon layer doped with N⁺ or P⁺ ions. After the metal layerand the doping layer are formed on the insulation substrate 300,photoresist is deposited on the doping layer and source and drainelectrodes 303 a and 303 b and doping layers 304 are simultaneouslyformed by etching the metal and doping layers in accordance with a maskprocess. Accordingly, doping patterns 305 are respectively formed overthe source and drain electrodes 303 a and 303 b.

After the source and drain electrodes 303 a and 303 b are formed on thegate insulation layer 302, a liquid-phase silicon layer is formed overand in between the source and drain electrodes 303 a and 303 b throughan inkjet method. The liquid-phase silicon layer is formed of a siliconcontaining liquid-phase material, such as Si_(x)H_(2x)(CyclopentaSilane). After the liquid-phase silicon layer is formedbetween the source and drain electrodes 303 a and 303 b, a channelpattern 304, as shown in FIG. 4B, is formed between the source and drainelectrodes 303 a and 303 b by etching the liquid-phase silicon layerthrough a photolithograph process, including a mask process. At thispoint, a portion of the doping pattern 305, which is not formed underedges of the channel pattern, is removed.

After the channel pattern 304 is formed between the source and drainelectrodes 303 a and 303 b, an annealing process is performed to form apolysilicon channel layer 304 a between the source and drain electrodes303 a and 303 b and an ohmic contact layers 306 at both edges of thechannel layer 304 a for connecting to the source and drain electrodes303 a and 303 b, as shown in FIG. 4C. As the annealing process isperformed, a thickness of the liquid-phase silicon layer is reduced anddopants doped in the ohmic contact layer 306 are diffused into bothedges of the channel layer 304 a. The annealing process is performed byheating the substrate up to a temperature of 200-800° C. (540° C.) andirradiating a laser having a wavelength of 308 nm and an energy of 345mJ/cm². In more detail, solvent contained in the channel pattern isremoved through the heating process and thus a thickness of the channelpattern is reduced. Further, the silicon is changed into polysilicon byirradiating the laser. However, embodiments of the present invention arenot limited to this configuration. The heating temperature and theenergy of the laser may vary in accordance with a degree ofcrystallization, or a size of the LCD device and a material property ofthe liquid-phase silicon. After the channel layer 304 a is formed, apassivation layer and terminals can be further formed.

The method of fabricating the TFT according to the third embodiment hasan advantage of forming the channel layer without performing depositionand mask processes. Further, since the channel layer is formed withoutperforming a PECVD process, the process load can be reduced.

FIGS. 5A through 5C are cross-sectional views illustrating a method offabricating a TFT according to a fourth embodiment of the presentinvention. As shown in FIG. 5A, a metal layer is formed on a transparentinsulation substrate 400 and etched to form a gate electrode 401, afterwhich a gate insulation layer 402 is formed on the insulation substrate400 covering the gate electrode 401. The gate insulation layer 402 is aninorganic insulation layer, such as SiNx or SiOx. Next, a metal layer isformed over the gate insulation layer 402 and a doping layer is formedon the metal layer. The doping layer may be a PSG layer, a BSG layer, oran amorphous silicon layer doped with N⁺ or P⁺ ions. After the metallayer and the doping layer are formed on the gate insulation layer 402,photoresist is deposited on the doping layer and source and drainelectrodes 403 a and 403 b and an ohmic contact layers 405 aresimultaneously formed by etching the metal and doping layers inaccordance with a mask process.

After the source and drain electrodes 403 a and 403 b are formed, aself-assembled monolayer (SAM) 410 is applied to the an ohmic contactlayers 405 and the gate insulation layer 402 between the source anddrain electrodes 403 a and 403 b. The SAM 410 has a hydrophilic orhydrophobic property. The property of the SAM 410 varies in accordancewith whether a liquid-phase silicon layer, which will be formed in thefollowing process has a hydrophilic or hydrophobic property.

After the SAM 410 is applied in the channel region between the sourceand drain electrodes 403 a and 403 b, a liquid-phase silicon layer isformed in the channel region and on the ohmic contact layers 405 througha coating process, such as an inkjet method, as shown in FIG. 5B. Theliquid-phase silicon layer is formed of silicon containing liquid-phasematerial, such as Si_(x)H_(2x) (CyclopentaSilane).

After the liquid-phase silicon layer is formed over and in between thesource and drain electrodes 403 a and 403 b and overlapping the ohmiccontact layers 405, the liquid-phase silicon layer exists only on aregion of the SAM 410 to form a channel pattern 404 a. When the SAM 410has a hydrophilic property and a liquid-phase silicon layer having ahydrophilic property is formed through a coating or inkjet method, theliquid-phase silicon layer exists only on the channel region to form thechannel pattern 404 a. After the channel pattern 404 a is formed, anannealing process is performed to form a channel layer 404, as shown inFIG. 5C. Although not shown in the drawings, as the annealing process isperformed, a thickness of the liquid-phase silicon layer is reduced anddopants doped in the ohmic contact layer 405 are diffused to the bothedges of the channel layer 404.

The annealing process is performed by heating the substrate up to atemperature of 200-800° C. (540° C.) and irradiating a laser having awavelength of 308 nm and an energy of 345 mJ/cm2. In more detail,solvent contained in the channel pattern is removed through the heatingprocess and thus a thickness of the channel pattern is reduced. Further,the silicon is changed into polysilicon by irradiating the laser.However, embodiments of the present invention are not limited to thisconfiguration. The heating temperature and the energy of the laser mayvary in accordance with a degree of crystallization, or a size of theLCD device and a material property of the liquid-phase silicon. Afterthe channel layer 404 is formed, a passivation layer and terminals canbe further formed.

The method of fabricating the TFT according to the fourth embodiment hasan advantage of forming the channel layer without performing depositionand masking processes. Further, since the channel layer is formedwithout performing a PECVD process, the process load can be reduced.

FIGS. 6A through 6C are cross-sectional views illustrating a method offabricating a TFT according to a fifth embodiment of the presentinvention. As shown in FIG. 6A, a metal layer is formed on a transparentinsulation substrate 500 and etched to form a gate electrode 501, afterwhich a gate insulation layer 502 is formed on the insulation substrate500 covering the gate electrode 501. The gate insulation layer 502 is aninorganic insulation layer, such as SiNx or SiOx.

Next, a metal layer is formed over the gate insulation layer 502 and adoping layer is formed on the metal layer. The doping layer may be a PSGlayer, a BSG layer, or an amorphous silicon layer doped with N+ or P+ions. After the metal layer and the doping layer are formed on the gateinsulation layer 502, photoresist is deposited on the doping layer andsource and drain electrodes 503 a and 503 b and doping layers 505 aresimultaneously formed by etching the metal and doping layers inaccordance with a mask process.

After the source and drain electrodes 503 a and 503 b are formed, aself-assembled monolayer (SAM) 510 is applied to edge portions of thedoping layers 505 away from the channel region between the source anddrain electrodes 503 a and 503 b and on the gate insulation layer 502outside of the channel area. The SAM 510 has a hydrophilic orhydrophobic property. The property of the SAM 510 varies in accordancewith whether a liquid-phase silicon layer, which will be formed in thefollowing process has a hydrophilic or hydrophobic property.

After the SAM 510 is applied outside of the channel region between thesource and drain electrodes 503 a and 503 b, a liquid-phase siliconlayer is formed in the channel region and on the ohmic contact layers505 through an inkjet method. The liquid-phase silicon layer is formedof a silicon containing liquid-phase material, such as SixH₂x(CyclopentaSilane).

After the liquid-phase silicon layer is formed in the channel regionbetween the source and drain electrodes 503 a and 503 b and overlappingthe ohmic contact layers 505, the liquid-phase silicon layer exists onlyon a region where the SAM 510 is not present to form a channel pattern504. For example, when the SAM 510 has a hydrophobic property, aliquid-phase silicon layer having a hydrophilic property is formedthrough a coating or inkjet method and thus the liquid-phase siliconlayer exists only on the region where the SAM 510 is not present to formthe channel pattern 504.

When the channel pattern 504 is formed in the channel region between thesource and drain electrodes 503 a and 503 b and overlapping the ohmiccontact layers 505, an annealing process is performed to form apolysilicon channel layer 504 a, as shown in FIG. 6C. Although not shownin the drawings, as the annealing process is performed, a thickness ofthe liquid-phase silicon layer is reduced and dopants doped in the ohmiccontact layer 505 are diffused at both edges of the channel layer 504 a.

The annealing process is performed by heating the substrate up to atemperature of 200-800° C. (540° C.) and irradiating a laser having awavelength of 308 nm and an energy of 345 mJ/cm². In more detail,solvent contained in the channel pattern is removed through the heatingprocess and thus a thickness of the channel pattern is reduced. Further,the silicon is changed into polysilicon by irradiating the laser.However, the present invention is not limited to this configuration. Theheating temperature and the energy of the laser may vary in accordancewith a degree of crystallization, or a size of the LCD device and amaterial property of the liquid-phase silicon. After the channel layer504 a is formed, a passivation layer and a power terminal can be furtherformed.

The method of fabricating the TFT according to the fifth embodiment hasan advantage of forming the channel layer without performing depositionand mask processes. Further, since the channel layer is formed withoutperforming a PECVD process, the process load can be reduced.

FIG. 7 is a top plan view of a pixel structure of a liquid crystaldisplay device according to a first embodiment of the present invention.As shown in FIG. 7, a gate line 601 for applying a driving signal and adata line 605 for applying a data signal are arranged to cross eachother to define a unit pixel area and a TFT is disposed at a regionwhere the gate line 601 crosses the data line 605. Since the TFT isformed by forming a liquid-phase silicon in a channel region through aninkjet method, the channel layer is formed between the source and drainelectrodes and overlapping the ohmic contact layers of the source anddrain electrodes.

A first common line 603 is formed at the unit pixel area. The firstcommon line 603 is in parallel with the gate line 601 and crosses thedata lien 605. A first common electrode 603 a extends from oppositesides of the common line 603 in parallel to the data line 605. A firstcommon line 603 is formed at the unit pixel area, and a first commonelectrode 603 a extends from opposite sides of the common line 603 inparallel to the data line 605. Here, the data line 605 and the firstcommon electrode 603 a are bent at a predetermined angle to provide awide viewing angle. Further, a first storage electrode 606 is formed ata region adjacent to the gate line 601 and the gate electrode 601 a andconnected to the first common electrode 603 a. Accordingly, the firststorage electrode 606 is integrally formed with the first common line603, the first common electrode 603 a and the first storage electrode606 to define a closed loop structure.

A second common line 613 is formed to overlap a central region of thefirst common line 603 formed at the unit pixel area and electricallyconnected to the first common line 603. The second common electrode 613a also extends from the second common line 613 along the unit pixelarea. Further, the second common electrode 613 a is also bent at apredetermined angle to be parallel with the first common electrode 603 aand the data line 605, thereby providing a wide viewing angle.

A second storage electrode 607 for forming a storage capacitance isformed above the first storage electrode 606 to overlap the firststorage electrode 606. First and second pixel electrodes 607 a and 607 bextend from the second storage electrode 607 to the unit pixel area.More particularly, the first pixel electrodes 607 a extend from thesecond storage electrode 607 and are alternately arranged with thesecond common electrodes 613 at a transmission area of the unit pixelarea. The first pixel electrodes 607 a are also bent at thepredetermined angle to thereby provide for a wide viewing angle.

The second pixel electrodes 607 b arranged at both edges of the pixelarea extend from the second storage electrode 607 and are disposed tooverlap the first common electrode 603 a extending from the first commonline 603. That is, the storage capacitance is formed between the firstand second storage electrodes 606 and 607 and another storagecapacitance is formed between the first common electrode 603 a and thesecond pixel electrode 607 b, thereby increasing the overall storagecapacitance. As described above, as the overall storage capacitanceincreases in the unit pixel area, the display quality improves.

The TFT of the first embodiment has an advantage of forming the channellayer without performing deposition and mask processes. Further, sincethe channel layer is formed without performing a PECVD process, theprocess load can be reduced.

FIGS. 8A through 8F are cross-sectional views taken along line I-I′ ofFIG. 7, illustrating a method of fabricating the liquid crystal displaydevice of FIG. 7. As shown in FIG. 8A, a metal layer is formed on atransparent insulating substrate 610 and the gate line (see FIG. 7),gate electrode 601 a, first common line (see FIG. 7), and first storageelectrode 606 are formed through a first mask process.

Then, a gate insulation layer 612 is formed over the gate line (see FIG.7), gate electrode 601 a, first common line (see FIG. 7), and firststorage electrode 606 on the insulation substrate 610. Subsequently,metal and doping layers are formed on the entire surface of the gateinsulation layer 612. The doping layer may be a PSG layer, a BSG layer,or an amorphous silicon layer doped with N+ or P+ ions. After the metallayer and the doping layer are formed, photoresist is deposited on thedoping metal layer. Source and drain electrodes 617 a and 617 b, anohmic contact layer 636, and the data line (not shown) aresimultaneously formed by etching the metal and doping layers inaccordance with a mask process, as shown in FIG. 8B. Accordingly, theohmic contact layers 636 are formed on the source and drain electrodes617 a and 617 b. At this time, the data line is formed to cross the gateline, thereby defining the pixel area.

After the source and drain electrodes 617 a and 617 b are formed, aliquid-phase silicon layer 633 is formed in a channel region definedbetween the source and drain electrodes 617 a and 617 b and on the ohmiccontact layers 636 through an inkjet method, as shown in FIG. 8C. Theliquid-phase silicon layer 633 is formed of silicon containing material,such as SixH₂x (CyclopentaSilane).

After the liquid-phase silicon layer 633 is formed in the channelregion, an annealing process is performed to form a channel layer 633 a,as shown in FIG. 8D. As the annealing process is performed, a thicknessof the liquid-phase silicon layer is reduced such that a height of thechannel layer 106 above the gate insulation layer 612 becomes similar tothat defined by the source and drain electrodes 617 a and 617 b and theohmic contact layers 636. The annealing process is performed by heatingthe substrate up to a temperature of 200-800° C. (540° C.) andirradiating a laser having a wavelength of 308 nm and an energy of 345mJ/cm². In more detail, solvent contained in the channel pattern isremoved through the heating process and thus a thickness of the channelpattern is reduced. Further, the silicon is changed into polysilicon byirradiating the laser. However, embodiments of the present invention arenot limited to this configuration. The heating temperature and theenergy of the laser may vary in accordance with a degree ofcrystallization, or a size of the LCD device and a material property ofthe liquid-phase silicon.

After the channel layer 633 a is formed, a passivation layer 619 isformed on the insulation substrate 610 and etched to expose a part ofthe ohmic contact layer 636 formed on the drain electrode 617 b, asshown in FIG. 8 e. The passivation layer 619 can be an inorganic ororganic layer. A transparent metal layer, such as an ITO layer, is thenformed on the passivation layer 619. Then, the second storage electrode607 and the first pixel electrode 607 a are formed through a maskprocess. More specifically, the second pixel electrode, second commonline and second common electrode that are shown in FIG. 7 are patternedtogether.

FIG. 9 is a top plan view of a pixel structure of a liquid crystaldisplay device according to a second embodiment of the presentinvention. As shown in FIG. 9, a gate line 701 for applying a drivingsignal and a data line 705 for applying a data signal are arranged tocross each other to define a unit pixel area. A TFT is disposed at aregion where the gate line 701 crosses the data line 705.

A first common line 703 is formed in the unit pixel area. The firstcommon line 703 is parallel with the gate line 701 and crosses the dataline 705. A first common electrode 703 a extends from opposite sides ofthe common line 703 parallel to the data line 705. The data line 705 andthe first common electrode 703 a are bent at a predetermined angle toprovide a wide viewing angle.

A first storage electrode 706 is formed in a region adjacent to the gateline 701 and the gate electrode 701 a and connected to the first commonelectrode 703 a. Accordingly, the first storage electrode 706 isintegrally formed with the first common line 703, the first commonelectrode 703 a and the first storage electrode 706 to define a closedloop structure. A second common line 713 is formed to overlap a centralregion of the first common line 703 formed in the unit pixel area andelectrically connected to the first common line 703. Further, the secondcommon electrode 713 a extends from the second common line 713. Thesecond common electrode 713 a is also bent at a predetermined angle tobe parallel with the first common electrode 703 a and the data line 705,thereby providing a wide viewing angle.

A second storage electrode 707 for forming a storage capacitance isformed above the first storage electrode 706 to overlap the firststorage electrode 706. First and second pixel electrodes 707 a and 707 bextend from the second storage electrode 707 to the unit pixel area.More particularly, the first pixel electrodes 707 a extend from thesecond storage electrode 707 and are alternately arranged with thesecond common electrodes 713 in a transmission area of the unit pixelarea. The first pixel electrodes 707 a are also bent at a predeterminedangle. The second pixel electrodes 707 b arranged at both edges of thepixel area extend from the second storage electrode 707 and are disposedto overlap the first common electrode 703 a extending from the firstcommon line 703. A storage capacitance is formed between the first andsecond storage electrodes 706 and 707 and another storage capacitance isformed between the first common electrode 703 a and the second pixelelectrode 707 b, thereby increasing the overall storage capacitance.

As described above, as the overall storage capacitance increases at theunit pixel area, the display quality can be improved. Further, since thechannel layer is formed without performing a PECVD process, the processload can be reduced. Furthermore, since the channel layer and the ohmiccontact layer are simultaneously formed after the source and drainelectrodes are formed, the fabrication process can be simplified.

FIGS. 10A through 10G are cross-sectional views taken along line II-II′of FIG. 9, illustrating a method of fabricating the liquid crystaldisplay device of FIG. 9. As shown in FIG. 10A, a metal layer is formedon a transparent insulating substrate 710. Then, a gate line, gateelectrode 701 a, first common line, and first storage electrode 706 areformed through a first mask process.

Then, a gate insulation layer 712 is formed over the gate line, gateelectrode 701 a, first common line, and first storage electrode 706. Ametal layer is then formed over the gate insulation layer 712.Subsequently, a photoresist is deposited on the metal layer and usedsimultaneously pattern the source electrode 717 a, drain electrode 717 band the data line (not shown) by etching the metal layer through aphotolithography process, as shown in FIG. 10B.

After the source and drain electrodes 717 a and 717 b are formed, aliquid-phase silicon layer 733 is formed over the doping layer 736 isformed on the liquid-phase silicon layer 733. The liquid-phase siliconlayer 733 is formed of silicon containing material, such as Si_(x)H_(2x)(CyclopentaSilane). The doping layer 736 may be a PSG layer, a BSGlayer, or an amorphous silicon layer doped with N⁺ or P⁺ ions.

After the liquid-phase silicon layer 733 and the doping layer 736 areformed, photoresist is deposited on the doping layer 736, after which ahalftone pattern 780 is formed at a channel region over the gateelectrode 701 a through a diffraction mask or halftone mask process.After the halftone pattern 780 is formed on the doping layer 736, asshown in FIG. 10C, an etching process is performed to form a channelpattern 733 a and an ohmic pattern 736 a that is partly overlapping thesource and drain electrodes 717 a and 717 b, as shown in FIG. 10D.

Subsequently, as shown in FIG. 10E, an annealing process and a contactlayer forming process using a laser are performed to form a channellayer 738 a on a region corresponding to the gate electrode 701 a and toform an ohmic contact layer 738 b on a region contacting the source anddrain electrodes 717 a and 717 b. The annealing process and the contactlayer forming process are performed by heating the substrate up to atemperature of 200-800° C. (about 540° C.) and irradiating a laserhaving a wavelength of 308 nm and an energy of 345 mJ/cm2. In moredetail, solvent contained in the channel pattern is removed through theheating process and thus a thickness of the channel pattern is reduced.Further, the silicon is changed into polysilicon by irradiating thelaser. In addition, during the above process, the doping layer isvertically diffused to the channel pattern to form the ohmic contactlayer 738 b.

Next, as shown in FIG. 10F, a passivation layer 719 is additionallyformed on the insulation substrate 710, after which a contact holeforming process for partly expose the drain electrode 717 b.

After the contact hole forming process is finished, as shown in FIG.10G, a transparent metal layer, such as an ITO layer, is formed. Thesecond storage electrode 707 and the first pixel electrode 707 a areformed through a mask process. At this point, the second pixelelectrode, second common line and second common electrode that are shownin FIG. 9 are patterned together.

The method for fabricating the LCD device is not limited to theabove-described embodiments. That is, the TFT fabrication methodsillustrated in FIGS. 4A through 4C, FIGS. 5A through 5C, and FIGS. 6Athrough 6C may be applied to the method for fabricating the LCD device.In addition, the above described TFT fabrication methods and LCDfabrication methods may be applied to a method of fabricating other flatdisplay devices as well as the LCD device.

According to embodiments of the present invention, an effect where thechannel layer and ohmic contact layer of the TFT are simultaneouslyformed can be obtained. In addition, the channel layer can be formedwithout performing deposition and mask processes so as to reduce processload reduced. Further, since the source and drain electrodes and theohmic contact layer are simultaneously patterned, the fabricationprocess can be simplified. Because the channel layer and the ohmiccontact layer of the TFT are simultaneously formed using theliquid-phase silicon and the halftone mask (refraction mask), thefabrication process can be simplified to reduce costs.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A thin film transistor comprising: a gate electrode; a gateinsulation layer on the gate electrode; source and drain metalelectrodes formed on the gate insulation layer; an ohmic contact layeron the source and drain metal electrodes; a polysilicon channel layer onthe gate insulation layer between the source and drain metal electrodes;and ohmic contact regions over the source and drain metal electrodes forcontacting the polysilicon channel to the source and drain metalelectrodes, wherein the ohmic contact layer is one of aphosphor-silicate-glass (PSG) layer or boro-silicate-glass layer (BSG).2. The thin film transistor according to claim 1, wherein thepolysilicon channel layer includes cyclopenta silane.
 3. The thin filmtransistor according to claim 1, wherein dopants doped in the ohmiccontact layer are diffused to the both edges of the polysilicon channellayer.